1. Technical Field
The present invention relates to a projection display device using laser light, and a speckle reduction element for reducing speckle noise.
2. Background Art
In recent years, as a high-output blue semiconductor laser has been commercialized, development of a projection display device incorporated with three primary color laser light sources has been progressed by using a red semiconductor laser, a green laser with a second harmonic generation device (hereinafter, abbreviated as “SHG device”), and the blue semiconductor laser. Use of a laser for emitting monochromatic laser light as a light source is advantageous in producing a projection display device with a wide reproducible color range, and a small electric power consumption. In the projection display device incorporated with the above laser light sources, generation of speckle noise has been a drawback. Speckle noise is a random interference pattern resulting from interference of laser light reflected on a screen having fine convex and concave portions in the case where an image is formed on the screen by using laser light having a high interference property. In a conventional projection display device, an interference pattern is changed at a high speed on a screen by mechanically moving a light diffuser to suppress detection of speckle noise by the human eye (see e.g. patent document 1).
FIG. 12 is a diagram showing an arrangement of a conventional projection display device recited in patent document 1. Referring to FIG. 12, laser light emitted from a red laser 101, a blue laser 103, and a green laser 105 is respectively wavelength-selectively reflected on dichroic mirrors 102, 104, and 106, and combined on a common optical axis. The combined laser light is diffused through a light diffusing element 107 made of ground glass or a like material, and thereafter, the diffused light illuminates a spatial light modulation element 110 via a lens 109. Laser light modulated by the spatial light modulation element 110 is projected on a screen 112 by a projection lens 111 as an image. The aforementioned interference pattern is changed at a high speed on the screen 112 by rotating the light diffusing element 107 by a motor 108. Thereby, speckle noise is eliminated.
In the arrangement of the conventional projection display device, since it is required to mechanically move the light diffusing element, an actuator such as the motor 108 is necessary. This may increase the size of the projection display device, or lower reliability of the device by wear of mechanical parts.    Patent document 1: Japanese Unexamined Patent Publication No. Hei 6-208089